The floodwaters and debris have receded in Horton Township near Renfrew, but Steve Osipenko hasn’t forgotten the frightening speed of last week’s massive landslide, and the flooding it caused.

Ten hectares of land slid into the Bonnechere River downstream from Renfrew on the night of March 28-29, clogging the fast-running river with trees and clay.

With the river blocked by trees and debris, water backed up behind the blockage. It rose more than seven metres near the landslide, and about five to six metres upstream in Renfrew.

Osipenko, who heads the Renfrew County Paramedic Service, could only watch and wait. Sooner or later, he said, “the river always finds a way through.”

But first the muddy floodwaters washed away a cottage, damaged a small hunting camp, poured water seven feet deep into a house, submerged the Renfrew sewage plant, and got into the basement of the hydroelectric plant.

Then after seven to eight hours the blockage finally broke loose, releasing all the debris into the Ottawa River.

As long as trees and mud blocked the river, a gauge downstream showed the rate of flow as zero. When the blockage burst, the flow jumped to 250 cubic metres of water per second — two and a half times the normal rate for a spring runoff.

The river is flowing almost normally now.

“There are definitely ancient, large landslides that are mapped in that area,” said Greg Brooks, a landslide expert with Natural Resources Canada. “This is another one.

He said the land includes a lot of Leda clay — the former bottom of the ancient Champlain Sea, which is prone to collapsing suddenly especially when it is sodden. Early spring while the ground is thawing is prime time for these landslides.

The size, though dramatic, isn’t a surprise, Brooks said.

“Sensitive clay landslides can easily be that size, or larger,” he said.

He recently studied a massive slide on the north side of the Ottawa River at Quyon. It took place about 1,000 years ago.

And east of Orléans there are former river channels with “very large, very old sensitive clay landslides.

“There certainly are major scars (old landslides) along the lower Bonnechere River between Renfrew and the Ottawa River.”

Leda clay has caused tragedy in this region.

In 1908 a slide in the middle of the night killed 34 people in Notre-Dame-de-la-Salette, north of Buckingham. The village of Lemieux, beside the South Nation River, had to be abandoned in the early 1990s because of a risk of landslides, and in 1993 a slide carried away 17 hectares of land from the abandoned site.